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TCP MSS clamping with iptables for IPSec tunnel

When routing traffic through a (IPSec) tunnel, an endpoint might need to do mss clamping if you are experiencing MTU issues.

For example, you are using a site-to-site VPN network, with a specific gateway as endpoint. When browsing websites through the tunnel, some websites might not load properly.

An example, using iptables to fix this problem:

iptables -A FORWARD -s 10.1.0.0/18 -o ens4 -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1360

This will set the mss to 1360 for traffic coming from 10.1.0.0/18 on interface ens4.

The 1360 value depends on the situation, 1360 bytes is the overhead created by IPsec encapsulation

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Multiple default gateways on Linux

Suppose you have a Linux machine doing IP forwarding (net.ipv4.ip_forward=1).

Depending on the incoming traffic, you might want to forward the packets to different gateways.

With just one gateway, you can simply add (or replace) the default gateway:
ip route add default via x.x.x.x

If you want to set a default gateway for a specific (incoming) IP range, you can add a custom routing table, using iproute2:

  • echo 200 custom >> /etc/iproute2/rt_tables
  • ip rule add from 10.1.2./0/24 table custom
  • ip route add default via y.y.y.y table custom
  • ip route flush cache